The present invention relates to an internal combustion engine with direct fuel injection, preferably a diesel engine, and concerns as an arrangement of the fuel injection nozzles to direct fuel spray past the valves for the cylinder.
Within vehicle engine technology there is ever greater demand for increased performance while at the same time increased environmental requirements are being set for emissions from engines. A major problem in this connection is that the combustion process is very complicated, so a measure adopted for a particular purpose often entails unintended disadvantages in another domain.
In direct injected diesel engines it has been necessary to compromise on various parameters in order to achieve acceptable combinations of performance and environmental acceptability. With regard to emission of nitrogen oxides it is undesirable to commence fuel injection too early. Nor is it desirable to increase the injection pressure overmuch, since this also adversely affects discharges of nitrogen oxides. On the other hand, it is undesirable to cease injection too late, since this increases the occurrence of undesirable residues of more or less unburnt fuel which may result in a variety of problems. If injection takes place too late, the engine""s piston will have passed its upper dead center point and will be so far down that instead of injection taking place into the hollow situated in the piston the fuel sprays arising from the injection are directed towards the walls of the cylinder. As the cylinder walls are significantly cooler than the temperature in the piston hollow, this causes the occurrence of uncombusted particles usually known collectively as soot. This means not only that the engine emits soot particles via its exhaust system but also that some of the soot particles will accumulate in the engine""s lubricating oil. This worsens the oil""s lubricating characteristics, thereby possibly resulting in serious wear damage to the engine. The presence of soot particles in exhaust gases makes it necessary to adopt special measures for preventing their release into the atmosphere, e.g. by using special soot traps or other post-treatment of exhaust gases. All these measures entail solutions which are complicated, space-consuming and expensive.
The foregoing shows that there is a problem in that the period of time available for injection is too limited. It is therefore difficult to effect fuel injection in such a way as to meet all the requirements and, in particular, to do so in such a way as to achieve both low emissions of nitrogen oxides and little soot formation.
One solution to the problem of fuel sprays reaching the cylinder walls might be to modify the so-called cone angle, which is the angle which the fuel sprays, viewed in a vertical plane, form with one another, so that even late injection is directed towards the piston hollow. This means, however, that at the commencement of injection the piston is too near to the injector and there is too little space between the injector and the piston for the fuel to become finely distributed before it reaches the piston. In a further-developed variant of that solution, it would be possible to envisage the initial stages of injection taking place with a certain cone angle and the final stages of injection taking place with a different cone angle. Such a solution would entail, however, exacting requirements with regard to injector control and result in an expensive and complicated solution.
Known technology which may cited with respect to the present invention is JP 59-173 554, which refers to a four-valve engine provided with fuel injector which creates various powerful fuel sprays in different directions. In that case the piston is provided not only with a conventional piston hollow but also with recesses for the valves and a larger amount of fuel is injected towards the valve recesses than the fuel sprays directed between the valve recesses. Despite in that case a smaller amount of fuel being directed between the valves, this means that these sprays will reach the cylinder wall if injection takes place late, with the result that the aforesaid soot formation problem persists.
The object of the invention is to provide a solution which brings about relatively late injection of the fuel so that the engine will not emit large amounts of nitrogen oxides, while at the same time the injection will not cause soot formation with its attendant disadvantages.
Another aim is to achieve this in a simple and inexpensive manner. A further aim is to be able to use the technique on existing engines.
The object of the invention is achieved by the fuel injector nozzle being so oriented in the cylinder that the projection of each fuel spray injection, viewed in a plane perpendicular to the cylinder center line, crosses a projection in the same plane of the valve disc of one of the valves. Orienting the fuel injector in this manner will cause the fuel spray to pass under a valve. Thermal radiation from the valve will cause more effective combustion and keep soot formation at a low level. As all the fuel sprays will be affected by valve heat, a more uniform environment for the various fuel sprays is achieved, resulting in a more homogeneous combustion. This is further assisted by the fact that a larger proportion of the fuel comes close to the hot valves. It will thus be possible to effect injection even at a time when the piston is relatively far from the injector and there would otherwise be risk of the fuel sprays reaching the cylinder wall.
Depending on the size of cylinder, it may be advantageous to have more than one fuel spray passing under one or more valves. In the case of a four-valve engine, for example, two fuel sprays may pass under each valve.
Further advantages and features of the invention are indicated in the description and the patent claims.